The field of art to which the invention pertains comprises the art of valves including liners and protectors therefor.
The prior art to which the invention is directed includes the art of valves and particularly butterfly valves being a type in which a usually circular closure vane or disc is mounted either centralized or offset for rotation in the body flow passage between an open and closed position. Construction for such valves commonly consists of a body and/or closure vane of cast iron or the like operable by means of an operator shaft extending through a bore in the body wall to externally thereof. Where the valve is likely to be utilized in corrosive service, the more corrosion vulnerable components and sometimes the entire valve is constructed of more exotic and more expensive materials such as stainless steel. As an alternative less costly construction, a recent trend in the industry for achieving corrosion protection has been to apply either a protective polymeric coating and/or a detachable liner about those components or surface portions susceptible to corrosive attack by the line content for which the valve is to be used. Exemplifying such prior art type butterfly valves for corrosive service of both the centralized and offset variety are U.S. Pat. Nos. 3,801,066; 3,563,510; 3,425,439; 3,376,014; 3,241,806; and 3,076,631.
Notwithstanding the added benefits which such protective coatings or liners afford, a problem which has thus far escaped a ready solution has been a satisfactory corrosion protection for the operator shafts per se. The problem is particularly acute with the offset or double offset variety of vane mounting in a butterfly valve as compared to a centralized mounting because of the increased exposure and vulnerability afforded the shafts in the former. Where a dynamic seal is employed to protect the shafts, the risk of leakage is minimized but not eliminated rendering the shafts ultimately vulnerable to attack. Yet, failure to protect the shafts in one form or another will eventually lead to destruction of the valve. For that reason and for lack of a less costly alternative, a frequent shaft construction for such service has been to utilize more costly and exotic corrosion resistant materials such as stainless steel.